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> Even if the glossary pointed to collections.html, 
> there are far more methods specified there than 
> are needed to be **able.
That is an implementation detail and is subject to
change. If someone supplies an argument satisfying
collections.Mapping, that should be sufficient across
all implementations.
> dict.update's doc string is explicit about what 
> it requires from the argument:
Duck-typing is still allowed when explicit requirements
have been exposed (we do this a lot with .readline() for
example). For the most part though, we want to specify
"needs a Mapping" in the sense of collections.Mapping.